FIGS. 16 and 17 are, respectively, an exploded perspective view of a conventional flat-type image display apparatus as disclosed in, for example, Japanese Unexamined Patent Publications Nos. 226949/1991 and 184239/1988 and an enlarged, partially broken away view of a principal part of another example. In FIGS. 16 and 17, numeral 1 denotes string like hot cathodes each of which is connected to a support and emits electrons when energized, and numeral 3 denotes perforated cover electrodes each covering each string like hot cathode 1 and each elliptically shaped in section. Each cover electrode 3 has small apertures permitting electrons to pass therethrough and serves to lead electrons out of the corresponding string like hot cathode 1 when an appropriate potential is applied. An electron-emitting source 40 comprises the string like hot cathodes 1, perforated cover electrodes 3 and back electrodes 42 fixing the cover electrodes 3 arranged in parallel and each assuming a potential equal to that of the corresponding cover electrode 3. Numeral 8 denotes a front glass which is coated on the inner surface thereof with three types of luminous elements (not shown) in a dotted or striped manner and with an aluminum film (not shown) covering these luminous elements for electric conduction, the luminous elements being adapted to be excited by electrons emitted from the electron-emitting source 40 to emit light in respective colors, i.e., red, green and blue.
In such a configuration, an application of a voltage of about 5 to about 30 kV to the aluminum film of the front glass 8 causes electrons to accelerate and thereby the luminous elements (not shown) to be excited to emit light. Numeral 4 denotes a control electrode for permitting or shutting off the passage of electrons led out by each cover electrode 3 toward the front glass 8, the control electrode being interposed between the front glass 8 and the string like hot cathodes 1. Numeral 10 denotes a focusing electrode to be applied with a predetermined voltage for causing an electron beam having passed through an electron-pass aperture 4a of the control electrode 4 to pass through an electron-pass aperture 10a and to be focused upon the corresponding dot of the luminous element. As shown in FIG. 17, the control electrode 4 includes a surface-insulative substrate 5 having electron-pass apertures 4a corresponding to pixels formed on the front glass 8, for example, a surface-insulative substrate 5 formed by coating an etch-perforated metallic substrate with an insulating film, a first control electrode group 6A including metallic electrodes 6 which are patterned like stripes having electron-pass apertures and arranged on the lower side of the surface-insulative substrate 5, or on the electron-emitting source 40 side, corresponding to respective columns of pixels, and a second control electrode group 7A, similar to the former group, including metallic electrodes 7 which are patterned like stripes having electron-pass apertures and arranged on the upper side of the surface-insulative substrate 5 corresponding to respective rows of pixels. Each metallic electrode of the first and second control electrode groups 6A and 7A is formed of, for example, a nickel film. The first and second control electrode groups 6A and 7A are insulated from each other by a nickel-free portion existing within each electron-pass aperture though nickel penetrates thereinto from both sides. The first control electrode group 6A further includes isolation grooves 44, or nickel-free insulating grooves, which extend between adjacent electron-pass apertures 4a in the direction perpendicular to the string like hot cathode 1. Similarly, the second control electrode group 7A includes isolation grooves 45 extending in the direction perpendicular to the isolation grooves 44 of the first control electrode group 6A. These are disposed within a sealed enclosure in the form of flat panel, the inside of which is kept in a vacuum state. Each of the electrodes is usually disposed in a flattened manner by means of a fix-hold member (not shown) and electrically connected to the outside through a seal portion provided on a lateral surface of the sealed enclosure.
FIGS. 18, 19(a) and 19(b) also illustrate another conventional flat-type image display apparatus. FIG. 19(a) is a perspective view of the arrangement of control electrodes 4 shown in FIG. 18, and FIG. 19(b) an enlarged view of a portion thereof. In FIGS. 18, 19(a) and 19(b) same numerals are used to denote corresponding parts of FIGS. 16 and 17, and the descriptions on such parts are omitted. This example has front glass 8 which has a curved shape and is so structured as to allow stress relaxation (to be described later) to be encouraged and the overall apparatus to be lightened. Further, as shown in FIGS. 19(a) and 19(b) this example has first control electrode group 6A and second control electrode group 7A which comprise stripe-like metallic electrodes 6 and stripe-like metallic electrodes 7, respectively, instead of the metallic film penetrating into electron-pass apertures of insulating substrate 5. These electrodes 6 and 7 are bonded to the insulating substrate 5 in such a manner that their electron-pass apertures are coincident with the corresponding electrode-pass apertures of the insulating substrate 5 thereby defining electron-pass apertures 4a of the control electrodes 4.
Description on the operation is as follows:
Thermoelectrons emitted from the string like hot cathode 1 are led out by a positive potential of about 5 to about 40 V applied to the perforated cover electrode 3, the positive potential being relative to the average potential of the string like hot cathode 1 assumed as a reference potential (hereinafter this average potential will be assumed to be 0 V). Further, by applying a positive potential of about 20 V to about 100 V relative to the potential of string like hot cathode 1 to one electrode of the first control electrode group 6A comprising metallic electrodes 6 arranged in the direction perpendicular to the string like hot cathode 1, the thermoelectons are drawn toward this electrode and reach the control electrode 4. This apparatus is designed such that the density of an electron beam on the surface of any metallic electrode of the first control electrode group 6A is made substantially uniform by adjusting the elliptical shape of the perforated cover electrode 3, the position of the first control electrode group 6A and the voltage to be applied to each metallic electrode 6.
It should be noted that although the operation of this control electrode 4 is not described in Japanese Unexamined Patent Publication No. 184239/1988, it is similar to the operation of typical matrix-type displays as disclosed in, for example, Japanese Unexamined Patent Publication Nos. 172642/1987, 126688/1989 and 226949/1991.
If only one metallic electrode 6 of the first control electrode group 6A assumes a positive potential (on-state) while the others assume 0 V or negative potential (off-state), the electrons emitted from the string like hot cathode 1 are attracted only by that control electrode 6 at a positive potential and enter each electrode-pass aperture 4a of this control electrode 6. However, not all the electrons entering the aperture pass therethrough toward the front glass 8. When the second control electrode group 7A is made to assume 0 V or negative potential, a negative potential region is produced by the second control electrode group 7A, so that the electrons stop within the electron-pass aperture 4a.
Consequently, electrons are allowed to pass through only the electron-pass aperture 4a lying at the intersection of the metallic electrode, being applied with a positive potential, of the first control electrode group 6A and the metallic electrode, being applied with a positive potential (for example 40 V to 100 V), of the second control electrode group 7A. The electrons having passed through the aperture 4a cause the luminous element positioned coincident with the aperture 4a to emit light. Therefore, a desired pixel display can be achieved by controlling the application of voltage to metallic electrodes 6 and 7 so that the intersection thereof is located corresponding to a desired position.
The luminance of each pixel is controlled by adjusting the on-state duration of each electrode of the second control electrode group 7A.
In this case, electron beam 2 (refer to FIG. 18) passing through the electron-pass aperture 4a is required to be focused within a phosphor dot corresponding to the aperture 4a. If the electron beam 2 passing dot of a luminous element through the electron-pass aperture 4a is incident on another too, color fringing will occur in the resulting image, or unclear contour of the image will result. For that reason, the focusing electrode 10 is provided for the purpose of controlling the path of the electron beam so that the electron beam is incident within a desired dot of a luminous element by applying an appropriate voltage to the focusing electrode.
Flat-type image display apparatus utilizing electron beam need to have electron-passing areas all kept in a vacuum and hence require a sealed vacuum enclosure. Where an image display apparatus is sold as a practical commercial article, for example, a television set for domestic use, it is desirable that the vacuum enclosure be made as light and thin (or small in the length in the direction perpendicular to the screen) as possible.
Where the above-mentioned conventional flat-type image display apparatus have a screen size as small as about 16 in., the glass thickness of the sealed enclosure does not need to be made so thick. However, where the screen size is as large as 20 in. or greater, the glass thickness needs to be made not smaller than about 20 mm to provide the enclosure with a sufficient strength against the vacuum. This results in a difficulty in reducing the weight of display apparatus of this type.
The most effective approach to lighten the vacuum enclosure is to shape the enclosure into a sphere ensuring the least stress concentration. However, this is against the demand mentioned above for a thinner enclosure. Assuming that the vacuum enclosure of a flat-type display apparatus is a box-like vacuum enclosure 11, as shown in section at FIG. 20(a), for accommodating an image display unit 11a, stress concentration caused by the pressure difference between the inside and outside of the vaccum enclosure 11 will occur at and angular portions and central portions of the screen indicated by arrows. If a reinforcing member is added to the enclosure at the angular portions or like portions to withstand such stress, the weight of the enclosure increases significantly. Therefore, the vacuum enclosure 11 in the form of oval in section as shown in FIG. 20(b) is most easily realized with the aim of making the enclosure 11 light and thin.
Typically, the phosphor for use in a television set is coated directly on the inner surface of the front glass forming part of the vacuum enclosure. This is because the provision of another glass plate or the like intermediate the front glass and the luminous element would cause light to reduce and hence the luminance of the display screen to decrease, because the screen would provide an unclear image even though the space between the front glass and the glass plate coated with the luminous element is placed in a vacuum state because the manufacturing cost is low, and like reasons.
In consequence, it is appreciated that the front glass of the vacuum enclosure needs to be shaped curved as having a curvature as shown in FIG. 18 so as to have a lightened weight and a reduced thickness, and that the luminous element is desirably coated on the inner surface of the front glass.
In the structure shown in FIG. 18, however, since the control electrode 4 and focusing electrode 10 are flat though the front glass is curved, there is a difference in the distance from the control electrode 4 or focusing electrode 10 to the front glass coated with the luminous element between end portion and central portion of the screen.
As described above, the focusing electrode 10 is applied with a desired voltage to focus electron beam 2 within a desired dot of luminous element. However, as shown in the enlarged sectional view at FIG. 21, where there is only one focusing electrode 10 and the voltage capable of being applied is fixed, the beam diameter reaches the minimum (becomes just focused) at only one point P.sub.1. Accordingly, where the distance Daf between the focusing electrode 10 and the front glass 8 provided with the aluminum film serving as the anode on the inner side thereof is uneven, it is impossible to cause the electron beam 2 to assume a minimum beam diameter at overall surface of the front glass 8. Stated otherwise, the beam diameter of electron beam 2 on the front glass is not fixed at different locations on the screen of the front glass 8 and, hence, the electron beam 2 becomes "fuzzy" at point P.sub.2 as shown in FIG. 18.
Where the electron beam 2 becomes "fuzzy", for example, the beam diameter of electron beam 2a exceeds the size of a pixel as shown in FIG. 22, black matrix 12 is also irradiated with the electron beam 2a, so that the intensity of the beam to be applied onto luminous element 9 decreases and, hence, the luminous intensity of the corresponding pixel decreases. Therefore, when the overall screen is viewed, luminance unevenness occurs Alternatively, where the electron beam 2 is an electron beam 2b having a beam diameter such that the beam extends from the subject pixel to pixels adjacent thereto over the pitch therebetween, the pixels other than the one desired to emit light are also caused to emit light, so that phenomena are developed such as color fringing and blurred contour of the resulting image.
Accordingly, when point P.sub.2 at which the electron beam 2 becomes "fuzzy" appears in a portion of the screen, luminance unevenness, color fringing or the like occurs at point P.sub.2. This is a fatal defect to a commercial article having a display screen.
To overcome such problem, there is disclosed in, for example, Japanese Unexamined Patent Publication No. 19947/1992 a structure wherein the wall of a sealed vaccum enclosure on light-emitting means side, the light-emitting means (phosphor-coated surface), an electron beam control electrode and an electron beam lead-out electrode are curved as having respective curvatures substantially equal to each other, while in addition a correction means is provided to render the quantity of electron beam incident on the electron beam lead-out electrode uniform in the horizontal direction; or a structure wherein the wall of the sealed vacuum enclosure on the light-emitting means side, a string like hot cathode, the electron beam lead-out electrode, electron beach control electrode and the light-emitting means are shaped into curved lines or curved surfaces as having respective curvatures substantially equal to each other. In this structure, however, the electron beam lead-out electrode cannot be applied to an electrode (perforated cover electrode) elliptically shaped in section and adapted to cover each string like hot cathode since the electron beam lead-out electrode is in the form of one plate and is a common electrode for all the string like hot cathodes, for avoiding the occurrence of substantial deformation thereof even though it is curved. In more detail, each perforated cover electrode needs to have a curvature small enough to form an elliptic section and further another curvature harmonizing with the curved surface of the front glass. In addition, the perforated cover electrode is located very near the string like hot cathode and hence is heated thereby to elevated temperatures, leading to severe thermal deformation. As a result, there arise problems that the luminance distribution on the display screen is possible to be extremely degraded, insulation failure between the perforated cover electrode and the cathode becomes likely, the life time of the cathode is shortened, and the like.